The invention relates to the manufacture of elongate articles comprising one or more elongate elements each loosely arranged within one or more channels or bores extending in the longitudinal direction of a carrier member, the diameter of the channel or bore being larger than that of the element.
Elongate article, carrier member or element is to be understood to mean a product the longitudinal dimension of which is much larger than the thickness or the diametrical dimension. The ratio length-diameter is at least 500 and will as a rule be 50,000 or more.
Such articles of large longitudinal dimensions can be manufactured, for example, by extruding a carrier member in the form of a separate tube around the elongate element. The element passes in a usual manner through a central aperture of a nozzle with which the separate tube is extruded around the element.
This known process has disadvantages if the element has a fragile character, because damage is apt to occur. Moreover, technologically, the process is less suitable or cannot be used for the manufacture of more complicated articles in which the article comprises many elements which are situated closely together but are separated from each other.
Due to the great importance of the method in accordance with the invention for the manufacture of optical cables or cable elements, the explanation hereinafter is directed mainly to the manufacture of optical cables and cable elements. It is emphasized that the method is not restricted hereto.
The manufacture of an optical cable or cable element presents the problem that an extremely fragile optical fibre has to be handled which is vulnerable both optically and mechanically. The optical fibre is normally a glass fibre which is coated with a thin so-called primary coating which is to protect the surface from damage. The fibre may also be a fibre of synthetic resin. The optical fibre exclusive of the primary coating has a diameter of approximately 125 /.mu.m and is very sensitive to fracture. Under the influence of mechanical stress the quality of the optical fibre can deteriorate considerably especially in the presence of moisture (stress corrosion).
In addition to the sensitivity to thermal and mechanical loads the optical fibre has the disadvantage that the thermal expansion differs considerably from that of the material with which the fibre is usually enveloped and protected during the manufacture of an optical cable. During the manufacture of an optical cable as well as in the manufacture of an electrical cable, synthetic resins are frequently used, in particular extrusile plastic synthetic resins whose coefficient of expansion is many times larger than that of the optical fibre. In the event of temperature variations the synthetic resin cladding of an optical fibre will show a much greater variation in length than the optical fibre itself. The differential in expansion may lead to an unacceptable deterioration in the optical and mechanical quality of the fibre. As a first self-supporting envelope of the optical fibre, a loose so-called secondary coating is therefore often used within which the optical fibre has some freedom of movement.
In a known and frequently used embodiment the optical cable element comprises an optical fibre and a loose coating of synthetic resin provided around the fibre by an extrusion process. As a result of the fragile character of the optical fibre the extrusion process must be carried out very carefully, and in particular stringent process parameters have to be observed. An additional disadvantage of the known process is that the high temperature of the extruded synthetic resin forms a thermal load for the optical fibre. In addition, upon cooling and solidification of the extruded coating, a considerable shrinkage will occur. After-shrinkage of the extruded coating also often occurs for a long period of time.
A number of the elementary cable elements thus manufactured may be twisted, for example, around a central strength member of metal or of a reinforced synthetic resin. The resulting elements may be provided with one or more protective coatings and/or be twisted to form larger elements or cables which in turn are provided with an outer sheath.
According to another known process of manufacturing a cable element, a synthetic resin-cladded aluminum foil is folded in a special manner so that in the longitudinal direction of the foil parallel open channels are formed having, for example, a cross-section in the form of a trapezoid. Optical fibres are laid in the channels and the foil is then covered with a second foil folded in the same manner, closed channels having cross-sections in the form of a regular hexagon being formed. The second foil may be connected to the first foil by means of glue or may be fused to the first foil by means of a thermal treatment. If desired, several of the cable elements thus formed may be stacked one on top of the other, an optical cable being formed which in cross-section has a honeycomb structure. The method is rather critical because the second foil must be accurately and carefully placed, glued, or bonded into the first foil. Moreover, the optical fibre should not be damaged.
An SZ form is a known and interesting configuration of an optical fibre. It is a spiral form having alternately left-hand and right-hand pitch. In still another known process of manufacturing an optical cable element the starting material is a central core of synthetic resin which may be reinforced, for example with metal. Grooves which extend in the longitudinal direction of the core and mutually have a spiral shape or SZ shape are provided in the surface of the synthetic resin. Optical fibres are laid in the grooves and the surface of the grooved core is then covered with an extruded sheath of synthetic resin. In this process also a thermal load of the optical fibre and a considerable shrinkage of the synthetic sheath occur.
The above-mentioned known processes have in common that the structure of the cable from the first stage, at the time that the first loose coating is provided, takes place in the presence of the very vulnerable optical fibre. This approach presents serious problems as they are stated by way of example in the preceding paragraph. Another serious disadvantage of the known processes is that it is very difficult or substantially impossible to realize a defined overlength of the optical fibre within the carrier member. A defined overlength of the optical fibre is of great importance, for example, upon bending an optical cable or cable element and for a good thermomechanical behaviour. Since it is necessary to expose the optical fibre as little as possible to thermal and mechanical loads, the process choice and the choice of the enveloping or coating material are furthermore very much restricted.
German Offenlegungsschrift 26 35 979 discloses an optical cable which comprises optical fibres or bundles of optical fibres within a protective sheath of metal. It appears from FIGS. 1 and 2 of the Offenlegungsschrift that the diameter of the protective sheath is very much larger than that of the optical fibres. Furthermore it is not clear in what manner the optical fibre shown in FIG. 1, in which a seamless protective sheath of metal is used, must be manufactured.
German Offenlegungsschrift 30 00 109 , to which U.S. Pat. No. 4,332,436 corresponds, discloses a process of manufacturing an optical cable in which one or more optical fibres are introduced in a capillary tube of metal by means of a flow of liquid. The process is inert with a rate of introduction of 5 meters per minute. The overall length of optical fibre which can be introduced is comparatively small. A length of 150 meters is mentioned on page 9. It is stated that larger lengths of optical fibre can be introduced in the case of very high liquid pressures. The high pressure is a serious disadvantage because expensive tubes of metal having large wall thicknesses have to be used. Moreover, the real danger exists that stress corrosion of the optical fibre occurs in the presence of moisture (water). Due to the necessary liquid flow the process is less suitable for practical applications. In this case also it holds that it is not possible to provide a defined overlength of optical fibre in the metal tube.